What a wave really does

Drop a stone in a pond and ripples spread out. These undulations (up-and-down movements) travel outwards — but the water itself does not travel with them.

A wave transfers energy from place to place without transferring matter.

• A floating cork bobs up and down as the ripple passes — it doesn't get carried to the edge.
• The energy moves across the pond; the water molecules stay roughly where they were.

Common mistake: waves do not carry the stuff along with them. Water waves move energy, not water. Sound waves move energy, not air across the room.

What does a wave carry?

The parts of a wave

Water and light waves are transverse: the vibrations are at right angles to the direction the wave travels. Every wave has an amplitude and a wavelength.

Watch out: the wavelength is a full cycle (crest to the next crest), not just crest-to-trough — that would only be half a wave.

Measuring a wavelength

When water waves meet

Water waves can be reflected. Ripples that hit the side of a pond bounce back off it.

When two waves meet, they add or cancel. This is called superposition.

• Crest meets crest: they add to make a bigger crest.
• Crest meets an equal trough: they cancel. Where they overlap, the water is flat.

Common mistake: waves do not bounce off each other. They pass through each other, adding or cancelling only where they overlap, then carry on.

When two crests meet

Frequency & hertz

The frequency of a wave is how many whole waves pass a point each second. It is measured in hertz (Hz).

• 1 Hz = one wave per second.
• 1000 Hz = a thousand waves per second (also written 1 kHz).

Both sound and light have a frequency. A high-frequency sound is a high-pitched note; a high-frequency light looks more towards the blue/violet end of the rainbow.

Link: for waves travelling at the same speed, more waves per second (higher frequency) means the waves are more squashed together — a shorter wavelength.

Sound needs a medium

Sound is made when something vibrates — a guitar string, a loudspeaker cone, your vocal cords. Those vibrations push and pull on the particles around them, sending a sound wave outwards.

• Sound waves are longitudinal: the particles vibrate back and forth in the same direction the wave travels, making squashed (compression) and stretched (rarefaction) regions.
• Sound needs a medium (a solid, liquid or gas) to travel through — it cannot travel through a vacuum, because there are no particles to pass the vibration on.

Sound in space

How fast does sound travel?

Sound is passed on from particle to particle. In a solid, the particles are close together and strongly held to their neighbours, so each vibration is passed on quickly. In a gas, the particles are far apart and hardly held at all. So sound is fastest in solids and slowest in gases:

Everyday clue: in a thunderstorm you see the lightning first and hear the thunder later. Mission 2 will show you why.

Echoes: reflected sound

When a sound wave hits a hard surface it can reflect back — the reflection you hear is an echo. Sound can also be:

• Reflected — bounced off a hard, smooth wall (giving echoes).
• Absorbed — soaked up by soft materials like carpet, curtains and foam (which is why a bare room echoes but a furnished one does not).

Hearing range & ultrasound

Sound waves make your ear drum vibrate; those vibrations are passed on and turned into signals your brain reads as sound.

• A healthy young human can hear from about 20 Hz to 20 000 Hz (20 kHz) — this is the hearing (auditory) range.
• Sound above 20 000 Hz is called ultrasound — too high for us to hear.
• Ultrasound is used for cleaning delicate objects, in physiotherapy, and for scans (like baby scans) — because it carries energy and reflects off surfaces inside the body.

Note: dogs and bats can hear much higher frequencies than humans — a dog whistle uses ultrasound that people can't hear.

Ear drum and microphone

We detect sound by its effect on something thin that can vibrate.

• In your ear, sound waves make the ear drum vibrate. The vibrations are passed on and turned into signals for your brain.
• In a microphone, sound waves make a thin diaphragm vibrate. The microphone turns these vibrations into an electrical signal.

The electrical signal carries the information in the sound, so it can be recorded or sent far away.

Watch out: the air does not flow into the microphone. Compressions and rarefactions arrive, and the diaphragm just vibrates back and forth.

Why would a space battle be silent?

Loudness & pitch

Sound waves are pressure waves that transfer energy. Two features of the wave decide how the sound seems to us:

• Amplitude → loudness. A bigger amplitude carries more energy, so the sound is louder.
• Frequency → pitch. A higher frequency (more waves per second) gives a higher-pitched note.

Common mix-up: amplitude is loudness (turning it up), frequency is pitch (high or low note). Turning the volume up does not change the pitch.

Turning up the volume

Property → effect

Tap a wave property on the left, then tap its effect on the right.

Why do you see lightning before you hear thunder?

Light travels in straight lines

Light is a wave too — but unlike sound, light can travel through a vacuum (which is why sunlight reaches us across empty space).

• Light travels in straight lines called rays — this is why you get sharp shadows.
• Light is extremely fast: about 300 000 000 m/s (300 million metres per second) — nothing goes faster.

Because light is so much faster than sound, you see distant events before you hear them — lightning before thunder (the flash and the thunder are made at the same moment, in the same place), a starting gun's flash before its bang.

The pinhole camera

A pinhole camera is a box with a tiny hole at the front and a screen of tracing paper at the back.

Light from each point on an object travels in a straight line through the hole to the screen. This makes an image.

• Rays from the top of the object land at the bottom of the screen, so the image is upside down.
• Drawing light as straight-line rays like this is called the ray model.

Watch out: there is no lens. The image is upside down because the rays travel in straight lines and cross at the hole.

See-through or not?

What happens when light hits a material?

• Transparent — light passes straight through (clear glass, water): you can see through it clearly.
• Translucent — some light gets through but is scattered (frosted glass, tissue paper): light comes through but you can't see a sharp image.
• Opaque — no light passes through (wood, metal, a book): it blocks light and casts a shadow.

Shadows form because light travels in straight lines and an opaque object blocks it — the light can't bend around to fill the gap.

The law of reflection

When light reflects off a mirror, we measure angles from the normal — an imaginary line at 90° to the surface.

Working out the angle

Mirror or paper?

At every point on a surface, reflected light obeys the law of reflection. What makes the difference is how smooth the surface is.

• Specular reflection: a smooth, shiny surface, like a mirror or still water. Parallel rays stay parallel, so you see a clear image.
• Diffuse reflection (scattering): a rough surface, like paper. The bumps face different ways, so the rays scatter in all directions. There is no image, but you can see the surface from anywhere.

Common mistake: paper does not break the law of reflection. Each ray still obeys it; the bumps just point different ways.

Why no face in the paper?

Bending at a boundary

When light passes from one material into another (say from air into glass or water), it changes speed and changes direction (unless it meets the boundary straight on, along the normal). This bending is called refraction.

• Going into a denser material (air → glass), light slows down and bends towards the normal.
• Coming back out (glass → air), it speeds up and bends away from the normal.

Everyday clue: a straw in a glass of water looks "broken" or bent at the surface — that's refraction bending the light on its way to your eye.

How we see: eye & camera

We see an object when light from it enters our eye. A convex lens refracts (bends) the light to focus a sharp image on the light-sensitive retina at the back.

• A camera works the same way: a lens focuses light onto a light-sensitive sensor (or film) instead of a retina.
• Light transfers energy from the source to the detector, causing a chemical or electrical effect that becomes a picture or a signal to the brain.

Key idea: you don't see by "shooting rays out of your eyes" — light comes into your eye from a source or a reflecting object.

Splitting white light

White light is really a mix of all the colours. A glass prism refracts each colour by a slightly different amount, spreading them into the visible spectrum.

Why do you see lightning before you hear thunder?

Absorb & reflect

An object's colour depends on which colours of light it reflects and which it absorbs:

• A red apple reflects red light and absorbs the other colours — so red reaches your eyes.
• A white object reflects all colours; a black object absorbs all colours (which is why black things heat up in the Sun).

Watch out: a red jumper under red light still looks red, but under pure blue light it looks nearly black — there's no red light for it to reflect.

Sound or light?

Tap a statement, then tap the wave it belongs to.

Your turn — refraction

The big ideas to remember

You've covered the key ideas across the four parts of the KS3 waves topic — observed waves, sound, energy & waves, and light. Press Finish to see your score.